This invention relates primarily to the field of clinical reference solutionsxe2x80x94quality control reagents and calibrators. More specifically it relates to methods of preparing multi-analyte reference solutions that have stable oxygen partial pressure (pO2) in zero headspace containers, preferably in flexible foil laminate containers. The solutions are stable at room temperature and have long shelf and use lives.
Clinical laboratories employ a variety of instrument systems for the analysis of patient samples. For example, pH/blood gas instruments measure blood pH, pCO2 and pO2. CO-Oximeter instruments typically measure the total hemoglobin concentration (tHb), and the hemoglobin fractionsxe2x80x94oxyhemoglobin (O2 Hb), carboxyhemoglobin (COHb), methemoglobin (MetHb), reduced hemoglobin (HHb) and sulfhemoglobin (SHb)(collectively referred to as xe2x80x9cCO-Ox fractionsxe2x80x9d). Ion selective electrode (ISE) instruments measure the content of blood electrolytes, such as, Na+, Clxe2x88x92, Ca++, K+, Mg++ and Li+. Also, a variety of other parameters such as, metabolites, e.g., glucose, lactate, creatinine and urea, can be measured in clinical laboratories by related instrument systems.
Instrument systems currently available may combine the measurement of blood pH, gases, electrolytes, various metabolites, and CO-Ox fractions in one instrument for a comprehensive testing of the properties of blood. For example, all such analytes are measured by the Rapidlab(trademark) 865 critical care diagnostics system from Chiron Diagnostics Corporation [Medfield, Mass. (USA)].
A calibrator is used to set the response level of the sensors. A control is used to verify the accuracy and reliability of such an instrumentation system.
A control is a solution having a known concentration of an analyte or analytes contained in the same, or a similar matrix in which the samples to be analyzed exist. The assay results from the control product are compared to the expected assay results to assure that the assay technique is performing as expected.
Commercial blood gas analysis systems have been available since the 1960s. The earliest reference materials were gas mixtures in pressurized cylinders, and those materials are still commonly used. In the 1970s, the development of liquid reference solutions began, leading to products in which reagents have been equilibrated with precision gas mixtures and packaged in flexible containers with zero headspace, requiring either refrigeration to maintain stability or the resort to calculations to compensate for the expected pO2 changes during storage.
Most quality control materials for such analyzers consist of tonometered aqueous solutions (a solution containing dissolved gases) in glass ampules. The typical gas headspace above the liquid in those ampules provides a reserve of oxygen against any potential oxygen-consuming reactions which may occur within the solution during the shelf life of the product.
In the absence of a gas headspace within their containers, reference solutions for oxygen determinations are particularly difficult to make and maintain stable. The inventors determined that the sources of said instability could be several.
First, the instability may be due to reactivity between the dissolved oxygen and the other components of the calibrator or quality control reagent. The other components might either react with the dissolved oxygen, reducing its concentration, or, alternatively, the other components may react with each other to generate oxygen, thus also changing the oxygen concentration. Second, the solution might be contaminated with microorganisms which, due to their metabolism, might change the oxygen content. Third, the oxygen might permeate through, or react with, the packaging material, also affecting the oxygen content of the reference material.
Reference materials that are manufactured for distribution in commerce must be made to withstand the various conditions encountered in the distribution chain and must be sufficiently stable to provide good performance within the time frame in which they are expected to be used by the customer, which is usually at least about six months, preferably for about nine months, and more preferably approximately 1 year for the typical calibrating or quality control solution distributed to commercial laboratories and hospitals. In addition, reference solutions, as with other reagents, should be packaged in containers which are easy to handle, convenient to use and which meet other design requirements of their intended usage. This is particularly true of reagents which are used in conjunction with various analytical instruments.
The users of instruments which determine the oxygen partial pressure of blood and other body fluids have a need for such reference materials and would benefit from liquid materials over the more conventional precision gas mixtures in cylinders with regulators. Liquid reference solutions are inherently less expensive, safer, and easier to manipulate than high-pressure gas tanks.
Although reference solutions used in instruments measuring pO2 have been made in the past, they have suffered from being unstable and having expensive, complicated, or unreliable means to access their contents. Some reference solutions, when used on analytical instruments, have extended their usefulness by allowing the instrument to calculate the expected oxygen level, said level being calculable from the age of the product, given the fact that the rate of decrease in oxygen level can be predicted based on historic performance [Conlon et al., Clin. Chem., 42: 6xe2x80x94Abstract S281 (1996)]. Several developers have included inner layers of plastic materials selected because of their heat sealability (e.g., U.S. Pat. No. 5,405,510xe2x80x94Betts) or low gas permeability (U.S. Pat. No. 4,116,336xe2x80x94Sorensen) or gas tightness (U.S. Pat. No. 4,163,734xe2x80x94Sorensen). Some have disclosed that the inner layer should be inert, but have not provided enablement as to how to select such an inner layer (U.S. Pat. No. 4,643,976xe2x80x94Hoskins) and/or weren""t capable of maintaining oxygen at a precise level appropriate for blood gas purposes.
Most blood gas/electrolyte/metabolite/CO-Oximetry/hematocrit quality controls (QCs) on the market today are provided in glass ampules which must be manually broken and manually presented to the analyzer. Ruther, H., U.S. Pat. No. 5,628,353 (issued May 13, 1997) describes an automated device which breaks open glass ampules by forcing a metal tube with thick walls and a small inner diameter, into the bottom of an ampule, and then aspirates the contents of the ampule into an analyzer. Such an automated ampule breaker is mechanically complex, requiring moving parts that are subject to wear and risk of failure, and could be subject to jamming and clogging from small bits of broken ampule glass.
In the 1980s, Kevin J. Sullivan disclosed an alternative to glass ampulesxe2x80x94the first commercial product with a blood gas reagent in a flexible, zero headspace package [U.S. Pat. Nos. 4,266,941; 4,375,743; and 4,470,520]. Coated aluminum tubes were filled with 40-50 mL of blood gas QC solutions without any headspace. The tubes were enclosed in pressurized cans, to prevent outgassing and to supply a source of force to cause the QC solutions to flow into the sample path of a blood gas analyzer. One container of Sullivan""s packaging design replaced about 30 glass ampules. Sullivan""s packaging relieved the user of the task of opening many glass ampules and of the attendent risks of broken glass. The disadvantages of Sullivan""s packaging included a need to refrigerate, a shelf life of less than a year, a menu of only three analytes, and the complexity and cost of a spring-loaded valve.
The instant invention not only overcomes the limitations of glass ampules, such as sensitivity of gas values to room temperature due to the headspace above the liquid, and complications resulting from the sharp edges which form upon breaking them open, or from the small, sharp glass pieces which can break off during ampule opening, but also overcomes the limitations of Sullivan""s zero headspace packaging described above. The multi-analyte reference solutions with stable pO2 of the instant invention are packaged in containers with zero headspace, preferably in flexible foil laminate containers, and are stable at room temperature for a shelf life of from about one to three years.
An additional shortcoming of storage devices for reference solutions for oxygen determinations (oxygen reference solutions) has been the opening or valve required to access the fluid for use, while maintaining the integrity of the fluid during storage. The materials available for valve construction and the need to breach the barrier layer to incorporate the valve may have compromised fluid stability. The access device disclosed herein for the preferred foil laminate containers used in the methods of the invention solves that problem. The simplicity of the one-piece valve should result in cost savings and greater reliability.
Further the multi-analyte reference solutions with stable pO2 in zero headspace containers of this invention provide cost savings in that one such container can be the equivalent of a box of 30 or more ampules that are currently on the market. Further cost savings are provided in the consolidation of formulations in 5 level quality control (QC) reagents of this invention which are useful to control from about 5 to about 20 analytes. Providing a reduced number of formulations to control for pH/blood gas/electrolyte/metabolite/total hemoglobin (tHb)/hematocrit and CO-Oximetry analytes saves time on an analyzer system, allowing for more patient samples to be assayed, and consequently minimizes assay costs.
One object of this invention was to overcome the shortcomings of glass ampules as storage containers for QCs and calibrators used with whole blood analyzers, while allowing for automation of QC and calibrator delivery. In one aspect, the instant invention overcomes problems presented by glass ampules as storage containers for oxygen reference solutions used as controls for instruments that measure blood analytes. Disclosed herein is a novel flexible package for oxygen reference solutions.
The package is made from a laminated film comprising an inner layer with low or no oxygen reactivity, preferably polypropylene, aluminum foil as the middle layer, and an outer layer that protects the aluminum foil layer from physical damage, e.g., abrasion or corrosion. The seams are heat sealed, while an optional access device for allowing access to the solution after the storage period, is attached to the inside wall of the bag without breaching the laminated layers. The foil laminate packaging allows for mechanical simplicity.
Preferred tubing for conveying a multi-analyte reference solution with stable pO2 from a container to a blood analyzer is also disclosed. Such tubing is flexible and relatively gas impervious, having a durometer (Shore D scale) in the range of 10 to 100, preferably from 70 to 94 and more preferably from 80 to 84. Preferred for such tubing are polyamide condensation polymers, more preferred are polyester/polyether block co-polymers or polyester elastomers, and especially preferred are Nylon(trademark) [DuPont; Wilmington, Del. (USA)] and Hytrel(trademark) 8238 [DuPont].
Another object of this invention is to provide multi-analyte reference solutions with stable pO2 in zero headspace containers, wherein the solutions are stable at room temperature for at least six months, preferably for at least nine months, more preferably for at least about a year, still more preferably for more than a year, and even more preferably for from two to up to three years. The most unstable component of a multi-analyte reference solution in a zero headspace environment used for oxygen determinations, among other analyses, is usually the pO2. Methods are provided to maintain the pO2 of such a multi-analyte reference solution within a predetermined range. Central to those methods is the principle of minimizing contact of the oxygen in the reference solution with materials that are oxygen reactive.
The lining of the preferred foil laminate packaging of this invention that contains the multi-analyte reference solutions with stable pO2 of this invention is selected for its low reactivity to oxygen. The preferred polypropylene lining of the foil laminate package, preferably a foil laminate pouch, was chosen as it is essentially inert to oxygen.
Further, source materials, particularly organic source materials, for the other components of the multi-analyte reference solutions with stable pO2 of this invention are also screened for low oxygen reactivity. It was found that some source materials contain impurities that are oxygen reactive enough to destabilize the pO2 of such multi-analyte reference solutions.
It is further an object of this invention to prepare a panel of multi-analyte reference solutions with stable pO2 that control from about 5 to about 20 analytes in as few containers as practicable, for example, a quality control reagent in five foil laminate containers (a 5 level QC reagent), wherein there is a different formulation in each zero headspace container. Key to combining so many critical analytes in as few containers as practicable are (1) using a low pH/low pO2/low glucose/low tHb formulation as an all-inclusive level; and (2) separating the mid-pO2 and high-pO2 reference solutions from glucose and from the dyes needed to simulate tHb and/or CO-Ox fractions.
A pH range considered low for the multi-analyte reference solutions of this invention is from about 6.4 to about 7.4. Exemplary of a low pO2 range is from about 20 mmHg to about 75 mmHg. Exemplary of a mid-pO2 to high pO2 range is from about 80 mmHg to about 600 mmHg. An exemplary low glucose concentration is from about 10 mg/dL to about 80 mg/dL. An exemplary low dye concentration corresponds to a tub concentration of from about 5 g/dL to about 11 g/dL.
Methods of preparing such reagents are disclosed as well as the reagents prepared by those methods. Further disclosed are representative embodiments of such a quality control reagent constituting five formulations (a 5 level QC reagent).
Although exemplified herein are uses for the multi-analyte reference solutions laminate with stable pO2 in zero headspace containers of this invention in the clinical field, they may also be used in the environmental and biotechnological fields, among other fields that require oxygen analysis. For example, the solutions of this invention would be useful in fermentation analyses.